Large data files often need to be protected against unauthorized access. As a result, various cryptographic techniques have been proposed and exploited. Many rely on secret binary keys. For example, a public-private cryptographic system has a private key known only to an individual or to a select group of people, and a public key which may be widely distributed. Data encoded according to the public key can be decoded only by those who know the private binary key. Other cryptographic systems are symmetric, and have a single key which is used both for encryption and decryption. In either case, however, managing the secret keys is problematic, especially in a situation where the keys are changed frequently.
Moreover, because the keys are uniquely associated with individuals or with small groups, the encryption and distribution of large files of data is a cumbersome process when the data needs to be accessed by a large number of independent users each having a different key. For example, consider the shooting and editing of a digital motion picture or film. A director in Hollywood may distribute each day's take to assistants at various locations. Because the resulting files have great value to would-be pirates, they need to be encrypted. Because each user has a different key, however, each file must be encrypted many times using the many different keys, and transmitted many times to the many different users, thus requiring massive processing power and communication bandwidth, and consuming a significant period of time. Further, distributing new keys to the multitude of users requires significant effort, and imposes a significant burden on the users themselves, who need to receive and install each update.
Thus there is a need for a method of encrypting large files of data to be accessed by a number of users, so that processing, bandwidth, delay, and key-management requirements are minimized.